Nanofitin as a New Molecular-Imaging Agent for the Diagnosis of Epidermal Growth Factor Receptor Over-Expressing Tumors

A Multispecific Checkpoint Inhibitor Nanofitin with a Fast Tumor Accumulation Property and Anti-Tumor Activity in Immune Competent Mice

Immune checkpoint inhibitors have revolutionized cancer treatment but remain limited by on-target/off-tumor effects that narrow their therapeutic window. Although PD-L1 is mainly expressed by tumor cells, these effects could reduce bloodstream availability and tumor accumulation of PD-L1 inhibitors. Enhancing tumor specificity through bispecific proteins targeting two tumor-associated antigens offers a promising strategy. This study evaluated a bispecific Nanofitin, B10-B11, targeting PD-L1 and EGFR. In vitro, B10-B11 efficiently bound to human A431 and murine CT26 cell lines, validating these models for in vivo studies. Nanofitins' accumulation in tumors and their anti-tumor efficacy were assessed, respectively, in A431 xenograft and CT26 immunocompetent mouse models. In both experiments, B10-B11 was compared with its albumin binding fused counterpart (B10-B11-ABNF). This study showed that the dual-targeting approach with the bispecific Nanofitin enhanced in vitro PD-L1 neutralization compared to the monomeric form and led to in vivo anti-tumor activity evidenced by reduced tumor growth and increased CD3+ T cells and F4/80+ macrophages in tumors. This activity was further correlated with Nanofitin's tumor accumulation at 7 h post-injection, which was highest for the B10-B11-ABNF. This study highlights the potential of bispecific Nanofitins, particularly with albumin binding to enable rapid and uniform tumor accumulation of effective PD-L1 immunotherapy.

Complete preclinical evaluation of the novel antibody mimetic Nanofitin-IRDye800CW for diverse non-invasive diagnostic applications in the management of HER-2 positive tumors

There are well-known limitations associated to the use of antibodies in the non-invasive detection of HER-2 expression. In fact, current procedures recommended for diagnostic purposes of HER-2 status are still invasive techniques. Here, a novel, smaller diagnostic probe, the anti-HER-2 Nanofitin conjugated to the fluorophore IRDye800CW (NF-800), underwent an in vitro/in vivo proof of concept study by Optical Imaging. NF-800 showed high affinity and specificity for the cellular target, and the ability to internalize into HER-2 positive cells. By ex vivo analysis, NF-800 showed a selective tumor accumulation in xenograft tumor models, and also a good tumor targeting efficacy in translational preclinical setups, such as orthotopic and patient-derived xenograft (PDX) models. In the latter, NF-800 was compared to the anti-HER-2 antibody Trastuzumab, displaying a large diagnostic advantage. Interestingly, NF-800 did not seem to share the same binding site with Trastuzumab and Pertuzumab, opening specific theragnostic opportunities for NF-800 in combination with standard-of-care antibodies.

Guiding Competitive Binding Assays Using Protein-Protein Interaction Prediction: The HER2-Affitin Use Case

Affitins are a class of small artificial proteins, designed as alternatives to antibodies for therapeutic, diagnostic, and biotechnological applications. Recent patents by Bracco Imaging S.p.A have demonstrated the potential of two engineered affitins for designing imaging probes to detect and monitor human epidermal growth-factor receptor 2 (HER2) levels in vivo. Targeting HER2 is critical, as its overexpression is linked to poor prognosis of several cancer diseases, making it a key marker for treatment strategies and diagnostic tools. Interestingly, these affitins do not compete with the commonly used monoclonal antibodies trastuzumab and pertuzumab for HER2 binding sites, allowing their concurrent use in vivo and making them suitable for imaging or diagnostic purposes. Since these two affitins compete for the same yet unidentified binding site on HER2, structural insights into these interactions are essential for facilitating the design and development of more effective diagnostic tools and treatments. In this study, we used protein-protein docking and molecular dynamics simulations to model the binding of these affitins to HER2. The stability of the predicted complexes was quantified by using the DockQ parameter, a widely used metric for evaluating protein-protein docking predictions. The docking poses were then compared with HER2 sites likely to interact with a protein partner, as predicted by the matrix of local coupling energies method. The combination of these two computational methods allowed for the identification of the most likely docking poses. Comparative analysis with HER2-protein complexes from the Protein Data Bank suggests that both affitins may bind HER2 at the same epitopes as an antibody fragment and an affibody. These findings indicate that targeted competitive binding assays could efficiently reduce the experimental efforts to map the HER2-affitin interactions. The computational approach proposed in this study not only provides insights into this specific case but also establishes a robust framework applicable for facilitating the structural modeling and interaction prediction of other affitin-protein systems.

Engineering the ADDobody protein scaffold for generation of high-avidity ADDomer super-binders

Adenovirus-derived nanoparticles (ADDomer) comprise 60 copies of adenovirus penton base protein (PBP). ADDomer is thermostable, rendering the storage, transport, and deployment of ADDomer-based therapeutics independent of a cold chain. To expand the scope of ADDomers for new applications, we engineered ADDobodies, representing PBP crown domain, genetically separated from PBP multimerization domain. We inserted heterologous sequences into hyper-variable loops, resulting in monomeric, thermostable ADDobodies expressed at high yields in Escherichia coli. The X-ray structure of an ADDobody prototype validated our design. ADDobodies can be used in ribosome display experiments to select a specific binder against a target, with an enrichment factor of ∼104-fold per round. ADDobodies can be re-converted into ADDomers by genetically reconnecting the selected ADDobody with the PBP multimerization domain from a different species, giving rise to a multivalent nanoparticle, called Chimera, confirmed by a 2.2 Å electron cryo-microscopy structure. Chimera comprises 60 binding sites, resulting in ultra-high, picomolar avidity to the target.

Enhancing Oral Delivery of Biologics: A Non-Competitive and Cross-Reactive Anti-Leptin Receptor Nanofitin Demonstrates a Gut-Crossing Capacity in an Ex Vivo Porcine Intestinal Model

Biotherapeutics exhibit high efficacy in targeted therapy, but their oral delivery is impeded by the harsh conditions of the gastrointestinal (GI) tract and limited intestinal absorption. This article presents a strategy to overcome the challenges of poor intestinal permeability by using a protein shuttle that specifically binds to an intestinal target, the leptin receptor (LepR), and exploiting its capacity to perform a receptor-mediated transport. Our proof-of-concept study focuses on the characterization and transport of robust affinity proteins, known as Nanofitins, across an ex vivo porcine intestinal model. We describe the potential to deliver biologically active molecules across the mucosa by fusing them with the Nanofitin 1-F08 targeting the LepR. This particular Nanofitin was selected for its absence of competition with leptin, its cross-reactivity with LepR from human, mouse, and pig hosts, and its shuttle capability associated with its ability to induce a receptor-mediated transport. This study paves the way for future in vivo demonstration of a safe and efficient oral-to-systemic delivery of targeted therapies.

Targeted Nanofitin-drug Conjugates Achieve Efficient Tumor Delivery and Therapeutic Effect in an EGFRpos Mouse Xenograft Model

Adjusting the molecular size, the valency and the pharmacokinetics of drug conjugates are as many leverages to improve their therapeutic window, notably by affecting tumor penetration, renal clearance, and short systemic exposure. In that regard, small tumor-targeting ligands are gaining attention. In this study, we demonstrate the benefits of the small Nanofitin alternative scaffolds (7 kDa) as selective tumor-targeting modules for the generation of drug conjugates, focusing on Nanofitins B10 and D8 directed against the EGFR. Owing to their small size and monovalent format, the two Nanofitins displayed a fast and deep tumor penetration in EGFR-positive A431 xenografts in BALB/c nude mice after intravenous administration, yielding to a targeting of respectively 67.9% ± 14.1 and 98.9% ± 0.7 of the tumor cells as demonstrated by IHC. Conjugation with the monomethyl auristatin E toxin provided homogeneous Nanofitin-drug conjugates, with an overall yield of ≥97%, for in vivo assessment in a curative xenograft model using bioluminescent, EGFR-positive, A431 cells in BALB/c nude mice. Internalization was found critical for efficient release of the toxin. Hence, the intravenous administration of the D8-based construct showed significant antitumor effect in vivo as determined by monitoring tumor volumes and bioluminescence levels over 2 months.

Inhalable Nanofitin demonstrates high neutralization of SARS-CoV-2 virus via direct application in respiratory tract

Nanofitins are small and hyperthermostable alternative protein scaffolds that display physicochemical properties making them suitable for the development of topical therapeutics, notably for the treatment of pulmonary infectious diseases. Local administration of biologics to the lungs involves a particularly stressful step of nebulization that is poorly tolerated by most antibodies, which limits their application by this delivery route. During the COVID-19 pandemic, we generated anti-SARS-CoV-2 monomeric Nanofitins of high specificity for the spike protein. Hit Nanofitin candidates were identified based on their binding properties with punctual spike mutants and assembled into a linear multimeric construction constituting of four different Nanofitins, allowing the generation of a highly potent anti-SARS-CoV-2 molecule. The therapeutic efficacy of the multimeric assembly was demonstrated both in in vitro and in vivo models. Interestingly, the neutralization mechanism of the multimeric construction seems to involve a particular conformation switch of the spike trimer. In addition, we reported the stability and the conserved activity of the tetrameric construction after nebulization. This advantageous developability feature for pulmonary administration associated with the ease of assembly, as well as the fast generation process position the Nanofitin technology as a potential therapeutic solution for emerging infectious diseases.

Engineering of a Bispecific Nanofitin with Immune Checkpoint Inhibitory Activity Conditioned by the Cross-Arm Binding to EGFR and PDL1

Re-education of the tumor microenvironment with immune checkpoint inhibitors (ICI) has provided the most significant advancement in cancer management, with impressive efficacy and durable response reported. However, low response rates and a high frequency of immune-related adverse events (irAEs) remain associated with ICI therapies. The latter can be linked to their high affinity and avidity for their target that fosters on-target/off-tumor binding and subsequent breaking of immune self-tolerance in normal tissues. Many multispecific protein formats have been proposed to increase the tumor cell’s selectivity of ICI therapies. In this study, we explored the engineering of a bispecific Nanofitin by the fusion of an anti-epidermal growth factor receptor (EGFR) and anti-programmed cell death ligand 1 (PDL1) Nanofitin modules. While lowering the affinity of the Nanofitin modules for their respective target, the fusion enables the simultaneous engagement of EGFR and PDL1, which translates into a selective binding to tumor cells co-expressing EGFR and PDL1 only. We demonstrated that affinity-attenuated bispecific Nanofitin could elicit PDL1 blockade exclusively in an EGFR-directed manner. Overall, the data collected highlight the potential of this approach to enhance the selectivity and safety of PDL1 checkpoint inhibition.

Selection of synthetic proteins to modulate the human frataxin function

Frataxin is a kinetic activator of the mitochondrial supercomplex for iron-sulfur cluster assembly. Low frataxin expression or a decrease in its functionality results in Friedreich's Ataxia (FRDA). With the aim of creating new molecular tools to study this metabolic pathway, and ultimately, to explore new therapeutic strategies, we have investigated the possibility of obtaining small proteins exhibiting a high affinity for frataxin. In this study, we applied the ribosome display approach, using human frataxin as the target. We focused on Affi_224, one of the proteins that we were able to select after five rounds of selection. We have studied the interaction between both proteins and discussed some applications of this specific molecular tutor, concerning the modulation of the supercomplex activity. Affi_224 and frataxin showed a K_D value in the nanomolar range, as judged by surface plasmon resonance analysis. Most likely, it binds to the frataxin acidic ridge, as suggested by the analysis of chemical shift perturbations (nuclear magnetic resonance) and computational simulations. Affi_224 was able to increase Cys NFS1 desulfurase activation exerted by the FRDA frataxin variant G130V. Importantly, Affi_224 interacts with frataxin in a human cellular model. Our results suggest quaternary addition may be a new tool to modulate frataxin function in vivo. Nevertheless, more functional experiments under physiological conditions should be carried out to evaluate Affi_224 effectiveness in FRDA cell models.

Evaluation of docking procedures reliability in affitins-partners interactions

Affitins constitute a class of small proteins belonging to Sul7d family, which, in microorganisms such as Sulfolobus acidocaldarius, bind DNA preventing its denaturation. Thanks to their stability and small size (60–66 residues in length) they have been considered as ideal candidates for engineering and have been used for more than 10 years now, for different applications. The individuation of a mutant able to recognize a specific target does not imply the knowledge of the binding geometry between the two proteins. However, its identification is of undoubted importance but not always experimentally accessible. For this reason, computational approaches such as protein-protein docking can be helpful for an initial structural characterization of the complex. This method, which produces tens of putative binding geometries ordered according to a binding score, needs to be followed by a further reranking procedure for finding the most plausible one. In the present paper, we use the server ClusPro for generating docking models of affitins with different protein partners whose experimental structures are available in the Protein Data Bank. Then, we apply two protocols for reranking the docking models. The first one investigates their stability by means of Molecular Dynamics simulations; the second one, instead, compares the docking models with the interacting residues predicted by the Matrix of Local Coupling Energies method. Results show that the more efficient way to deal with the reranking problem is to consider the information given by the two protocols together, i.e. employing a consensus approach.

Ex silico engineering of cystine-dense peptides yielding a potent bispecific T cell engager

Cystine-dense peptides (CDPs) are a miniprotein class that can drug difficult targets with high affinity and low immunogenicity. Tools for their design, however, are not as developed as those for small-molecule and antibody drugs. CDPs have diverse taxonomic origins, but structural characterization is lacking. Here, we adapted Iterative Threading ASSEmbly Refinement (I-TASSER) and Rosetta protein modeling software for structural prediction of 4298 CDP scaffolds and performed in silico prescreening for CDP binders to targets of interest. Mammalian display screening of a library of docking-enriched, methionine and tyrosine scanned (DEMYS) CDPs against PD-L1 yielded binders from four distinct CDP scaffolds. One was affinity-matured, and cocrystallography yielded a high-affinity (K_D = 202 pM) PD-L1-binding CDP that competes with PD-1 for PD-L1 binding. Its subsequent incorporation into a CD3-binding bispecific T cell engager produced a molecule with pM-range in vitro T cell killing potency and which substantially extends survival in two different xenograft tumor-bearing mouse models. Both in vitro and in vivo, the CDP-incorporating bispecific molecule outperformed a comparator antibody-based molecule. This CDP modeling and DEMYS technique can accelerate CDP therapeutic development.

Discovery of APL-1030, a Novel, High-Affinity Nanofitin Inhibitor of C3-Mediated Complement Activation

Uncontrolled complement activation contributes to multiple immune pathologies. Although synthetic compstatin derivatives targeting C3 and C3b are robust inhibitors of complement activation, their physicochemical and molecular properties may limit access to specific organs, development of bifunctional moieties, and therapeutic applications requiring transgenic expression. Complement-targeting therapeutics containing only natural amino acids could enable multifunctional pharmacology, gene therapies, and targeted delivery for underserved diseases. A Nanofitin library of hyperthermophilic protein scaffolds was screened using ribosome display for C3/C3b-targeting clones mimicking compstatin pharmacology. APL-1030, a recombinant 64-residue Nanofitin, emerged as the lead candidate. APL-1030 is thermostable, binds C3 (K_D, 1.59 nM) and C3b (K_D, 1.11 nM), and inhibits complement activation via classical (IC₅₀ = 110.8 nM) and alternative (IC₅₀ = 291.3 nM) pathways in Wieslab assays. Pharmacologic activity (determined by alternative pathway inhibition) was limited to primate species of tested sera. C3b-binding sites of APL-1030 and compstatin were shown to overlap by X-ray crystallography of C3b-bound APL-1030. APL-1030 is a novel, high-affinity inhibitor of primate C3-mediated complement activation developed from natural amino acids on the hyperthermophilic Nanofitin platform. Its properties may support novel drug candidates, enabling bifunctional moieties, gene therapy, and tissue-targeted C3 pharmacologics for diseases with high unmet need.

Protein scaffolds: Antibody alternative for cancer diagnosis and therapy

Although antibodies are well developed and widely used in cancer therapy and diagnostic fields, some defects remain, such as poor tissue penetration, long in vivo metabolic retention, potential cytotoxicity, patent limitation, and high production cost. These issues have led scientists to explore and develop novel antibody alternatives. Protein scaffolds are small monomeric proteins with stable tertiary structures and mutable residues, which emerged in the 1990s. By combining robust gene engineering and phage display techniques, libraries with sufficient diversity could be established for target binding scaffold selection. Given the properties of small size, high affinity, and excellent specificity and stability, protein scaffolds have been applied in basic research, and preclinical and clinical fields over the past two decades. To date, more than 20 types of protein scaffolds have been developed, with the most frequently used being affibody, adnectin, ANTICALIN®, DARPins, and knottin. In this review, we focus on the protein scaffold applications in cancer therapy and diagnosis in the last 5 years, and discuss the pros and cons, and strategies of optimization and design.

Although antibodies are well developed and widely used in cancer therapy and diagnostic fields, some defects remain, such as poor tissue penetration, long in vivo metabolic retention, potential cytotoxicity, patent limitation, and high production cost.

Recent Advances in the Scaffold Engineering of Protein Binders

In recent years, extensive attention has been given to the generation of new classes of ligand- specific binding proteins to supplement monoclonal antibodies. A combination of protein engineering and display technologies has been used to manipulate non-human antibodies for humanization and stabilization purposes or even the generation of new binding proteins. Engineered protein scaffolds can now be directed against therapeutic targets to treat cancer and immunological disorders. Although very few of these scaffolds have successfully passed clinical trials, their remarkable properties such as robust folding, high solubility, and small size motivate their employment as a tool for biology and applied science studies. Here, we have focused on the generation of new non-Ig binding proteins and single domain antibody manipulation, with a glimpse of their applications.

Nanofitins targeting heat shock protein 110: An innovative immunotherapeutic modality in cancer

The presence of an inactivating heat shock protein 110 (HSP110) mutation in colorectal cancers has been correlated with an excellent prognosis and with the ability of HSP110 to favor the formation of tolerogenic (M2-like) macrophages. These clinical and experimental results suggest a potentially powerful new strategy against colorectal cancer: the inhibition of HSP110. In this work, as an alternative to neutralizing antibodies, Nanofitins (scaffold ~7 kDa proteins) targeting HSP110 were isolated from the screening of a synthetic Nanofitin library, and their capacity to bind (immunoprecipitation, biolayer interferometry) and to inhibit HSP110 was analyzed in vitro and in vivo. Three Nanofitins were found to inhibit HSP110 chaperone activity. Interestingly, they share a high degree of homology in their variable domain and target the peptide-binding domain of HSP110. In vitro, they inhibited the ability of HSP110 to favor M2-like macrophages. The Nanofitin with the highest affinity, A-C2, was studied in the CT26 colorectal cancer mice model. Our PET/scan experiments demonstrate that A-C2 may be localized within the tumor area, in accordance with the reported HSP110 abundance in the tumor microenvironment. A-C2 treatment reduced tumor growth and was associated with an increase in immune cells infiltrating the tumor and particularly cytotoxic macrophages. These results were confirmed in a chicken chorioallantoic membrane tumor model. Finally, we showed the complementarity between A-C2 and an anti-PD-L1 strategy in the in vivo and in ovo tumor models. Overall, Nanofitins appear to be promising new immunotherapeutic lead compounds.

Review: PET imaging with macro- and middle-sized molecular probes

Recent progress in radiolabeling of macro- and middle-sized molecular probes has been extending possibilities to use PET molecular imaging for dynamic application to drug development and therapeutic evaluation. Theranostics concept also accelerated the use of macro- and middle-sized molecular probes for sharpening the contrast of proper target recognition even the cellular types/subtypes and proper selection of the patients who should be treated by the same molecules recognition. Here, brief summary of the present status of immuno-PET, and then further development of advanced technologies related to immuno-PET, peptidic PET probes, and nucleic acids PET probes are described.

Characterization of Affitin proteolytic digestion in biorelevant media and improvement of their stabilities via protein engineering

Affitins are a novel class of small 7 kDa artificial proteins which can be used as antibody substitutes in therapeutic, diagnostic and biotechnological applications. One challenge for this type of protein agent is their behaviour in the context of oral administration. The digestive system is central, and biorelevant media have fast emerged as relevant and reliable tools for evaluating the bioavailability of drugs. This study describes, for the first time, the stability of Affitins under simulated gastric and intestinal digestion conditions. Affitins appear to be degraded into stable fragments in in vitro gastric medium. We identified cleavage sites generated by pepsin that were silenced by site-directed mutagenesis. This protein engineering allowed us to enhance Affitin properties. We showed that a mutant M1 containing a double mutation of amino acid residues 6 and 7 in H4 and C3 Affitins acquired a resistance against proteolytic digestion. In addition, these mutations were beneficial for target affinity, as well as for production yield. Finally, we found that the mutated residues kept or increased the important pH and temperature stabilities of Affitins. These improvements are particularly sought after in the development of engineered binding proteins for research tools, preclinical studies and clinical applications.

[Ribosome display: Evolution and acellular selection of molecular libraries for high affinity binder generation]

Ribosome display is a powerful method for selection and molecular evolution of proteins and peptides from large libraries. Displayed proteins are recovered from target molecules in multiple rounds of selection in order to enrich specific binders with the desired properties. Nowadays, ribosome display has become one of the most widely-used display technologies thanks to its advantages over cell-display as phage display. Ribosome display is an in vitro method, in which a stable ternary complex is formed between the mRNA, the ribosome and the nascent protein. A selection cycle can be performed in a few days and bacterial transformation is not necessary. Ribosome display has been used to screen and select peptides, proteins or molecular scaffolds in order to increase their affinity, specificity, catalytic activity or stability. In this review, ribosome display systems and their applications in selection and evolution of proteins are described.

Miniproteins as a Powerful Modality in Drug Development

Miniproteins are a diverse group of protein scaffolds characterized by small (1-10 kDa) size, stability, and versatility in drug-like roles. Coming largely from native sources, they have been widely adopted into drug development pipelines. While their structures and capabilities are diverse, the approaches to their utilization share more similarities with each other than with more widely used modalities (e.g., antibodies or small molecules). In this review, we highlight recent advances in miniprotein-based approaches to otherwise poorly addressed clinical needs, including structure-based and functional characterization. We also summarize their unique screening strategies and pharmacology considerations. Through a greater understanding of the unique properties that make them attractive for drug design, miniproteins can be effectively utilized against targets that are intractable by other approaches.

Les formats alternatifs aux anticorps

Les anticorps sont désormais devenus d’une utilisation courante dans un large champ thérapeutique qui n’est plus restreint à la cancérologie et à l’inflammation. Cette explosion du domaine conduit à des besoins nouveaux qui peuvent être mieux remplis par des molécules inspirées mais différentes des anticorps classiques. En particulier, la molécule anticorps a de multiples fonctions qui ne sont pas toujours nécessaires, comme sa capacité à recruter les cellules du système immunitaire, à se lier de façon bivalente à sa cible ou à présenter une demi-vie plasmatique élevée. En revanche, dans la grande majorité des applications, sa remarquable capacité à reconnaître spécifiquement sa cible moléculaire et surtout sa diversité de reconnaissance doivent être conservées. De plus, les anticorps sont des molécules de très haut poids moléculaire, coûteuses à produire et qui présentent des propriétés physicochimiques limitées ne permettant pas leur utilisation dans des milieux agressifs. Finalement, dans certaines applications thérapeutiques, la grande taille de la molécule (environ 150 kDa) peut également limiter sa diffusion dans les tissus et empêcher la reconnaissance de certaines structures moléculaires peu accessibles. Pour répondre à ces limitations, de nombreux formats alternatifs aux anticorps entiers ont été développés au cours de ces vingt dernières années. Les applications couvrent les domaines de la biotechnologie, du diagnostic in vitro et in vivo et de la thérapie. Deux grandes familles de molécules permettent de couvrir ce champ et seront présentées dans cette mini-revue. Une première famille s’appuie sur la diversité naturelle des anticorps mais en en réduisant la taille, comme les fragments d’anticorps classiques (Fab, scFv) ou ceux provenant des camélidés ou des requins (VHH, V-NAR). La deuxième famille a été développée en partant des propriétés finales désirées et notamment la stabilité en milieu extrême et la productivité en système simple et économique de production comme l’utilisation de bactéries et en y greffant des propriétés de liaison comparables aux anticorps par des méthodes d’évolution moléculaire dirigée in vitro. Cette mini-revue se concentrera sur les molécules les plus avancées, mais le domaine est en très forte et rapide expansion. Il faut noter que beaucoup de ces molécules, voire ces approches, sont couvertes par des brevets et sont souvent développées dans le cadre de jeunes sociétés innovantes dont certaines ont déjà été rachetées par de grands groupes de la pharmacie.

PET Imaging of Receptor Tyrosine Kinases in Cancer

Overexpression and/or mutations of the receptor tyrosine kinase (RTK) subfamilies, such as epidermal growth factor receptors (EGFR) and vascular endothelial growth factor receptors (VEGFR), are closely associated with tumor cell growth, differentiation, proliferation, apoptosis, and cellular invasiveness. Monoclonal antibodies (mAb) and tyrosine kinase inhibitors (TKI) specifically inhibiting these RTKs have shown remarkable success in improving patient survival in many cancer types. However, poor response and even drug resistance inevitably occur. In this setting, the ability to detect and visualize RTKs with noninvasive diagnostic tools will greatly refine clinical treatment strategies for cancer patients, facilitate precise response prediction, and improve drug development. Positron emission tomography (PET) agents using targeted radioactively labeled antibodies have been developed to visualize tumor RTKs and are changing clinical decisions for certain cancer types. In the present review, we primarily focus on PET imaging of RTKs using radiolabeled antibodies with an emphasis on the clinical applications of these immunoPET probes. Mol Cancer Ther; 17(8); 1625-36. ©2018 AACR.

Analysis of Progress and Challenges of EGFR-Targeted Molecular Imaging in Cancer With a Focus on Affibody Molecules

Epidermal growth factor receptor (EGFR)-targeted cancer therapy requires an accurate estimation of EGFR expression in tumors to identify responsive patients, monitor therapeutic effect, and estimate prognosis. The EGFR molecular imaging is an optimal method for evaluating EGFR expression in vivo accurately and noninvasively. In this review, we discuss the recent advances in EGFR-targeted molecular imaging in cancer, with a special focus on the development of imaging agents, including epidermal growth factor (EGF) ligand, monoclonal antibodies, antibody fragments, Affibody, and small molecules. Each substrate or probe, whether it is an endogenous ligand, antibody, peptide, or small molecule labeled with fluorochrome or radionuclide, has unique advantages and limitations. Antibody-based probes have high affinity but a long metabolic cycle and therefore offer poor imaging quality. Affibody molecules promise to surpass antibody-based probes due to their small size, stable chemical properties, and high affinity to the target. Small-molecule probes are safe, have favorable pharmacokinetics, and show high affinity and specificity, in addition to having an ideal size, but are inadequate for delayed imaging after injection due to their fast clearance.

A Novel Engineered Small Protein for Positron Emission Tomography Imaging of Human Programmed Death Ligand-1: Validation in Mouse Models and Human Cancer Tissues

PURPOSE

To design and evaluate a small engineered protein binder targeting human programmed death-1 ligand (hPD-L1) in vivo for PET imaging in four mouse tumor models, and in situ in human cancer specimens.Experimental Design: The hPD-L1 protein binder, FN3_hPD-L1, was engineered using a 12-kDa human fibronectin type-3 domain (FN3) scaffold. The binder's affinity was assayed in CT26 mouse colon carcinoma cells stably expressing hPD-L1 (CT26/hPD-L1). ⁶⁴Cu-FN3_hPD-L1 was assayed for purity, specific activity, and immunoreactivity. Four groups of NSG mice (n = 3-5/group) were imaged with ⁶⁴Cu-FN3_hPD-L1 PET imaging (1-24 hours postinjection of 3.7 MBq/7 μg of Do-FN3 in 200 μL PBS): Nod SCID Gamma (NSG) mice bearing (i) syngeneic CT26/hPD-L1tumors, (ii) CT26/hPD-L1 tumors blocked (blk) by preinjected nonradioactive FN3_hPD-L1 binder, (iii) hPD-L1-negative Raji xenografts, and (iv) MDA-MB-231 xenografts. The FN3_hPD-L1 binder staining was evaluated against validated hPD-L1 antibodies by immunostaining in human cancer specimens.

RESULTS

FN3_hPD-L1 bound hPD-L1 with 1.4 ± 0.3 nmol/L affinity in CT26/hPD-L1 cells. ⁶⁴Cu-FN3_hPD-L1 radiotracer showed >70% yield and >95% purity. ⁶⁴Cu-FN3_hPD-L1 PET imaging of mice bearing CT26/hPD-L1 tumors showed tumor-to-muscle ratios of 5.6 ± 0.9 and 13.1 ± 2.3 at 1 and 4 hours postinjection, respectively. The FN3_hPD-L1 binder detected hPD-L1 expression in human tissues with known hPD-L1 expression status based on two validated antibodies.

CONCLUSIONS

The ⁶⁴Cu-FN3_hPD-L1 radiotracer represents a novel, small, and high-affinity binder for imaging hPD-L1 in tumors. Our data support further exploration and clinical translation of this binder for noninvasive identification of cancer patients who may respond to immune checkpoint blockade therapies.

Imaging of Tumor Characteristics and Molecular Pathways With PET: Developments Over the Last Decade Toward Personalized Cancer Therapy

PURPOSE

Improvements in personalized therapy are made possible by the advances in molecular biology that led to developments in molecular imaging, allowing highly specific in vivo imaging of biological processes. Positron emission tomography (PET) is the most specific and sensitive imaging technique for in vivo molecular targets and pathways, offering quantification and evaluation of functional properties of the targeted anatomy.

MATERIALS AND METHODS

This work is an integrative research review that summarizes and evaluates the accumulated current status of knowledge of recent advances in PET imaging for cancer diagnosis and treatment, concentrating on novel radiotracers and evaluating their advantages and disadvantages in cancer characterization. Medline search was conducted, limited to English publications from 2007 onward. Identified manuscripts were evaluated for most recent developments in PET imaging of cancer hypoxia, angiogenesis, proliferation, and clonogenic cancer stem cells (CSC).

RESULTS

There is an expansion observed from purely metabolic-based PET imaging toward antibody-based PET to achieve more information on cancer characteristics to identify hypoxia, proangiogenic factors, CSC, and others. ⁶⁴Cu-ATSM, for example, can be used both as a hypoxia and a CSC marker.

CONCLUSIONS

Progress in the field of functional imaging will possibly lead to more specific tumor targeting and personalized treatment, increasing tumor control and improving quality of life.

Antibody-drug conjugates: Design and development for therapy and imaging in and beyond cancer, LabEx MAbImprove industrial workshop, July 27-28, 2017, Tours, France

The annual "Antibody Industrial Symposium", co organized by LabEx MAbImprove, MabDesign and Polepharma, was held in Tours, France on June 27-28, 2017. The focus was on antibody-drug-conjugates (ADCs), new entities which realize the hope of Paul Ehrlich's magic bullet. ADCs result from the bioconjugation of a highly cytotoxic drug to a selective monoclonal antibody, which acts as a vector. Building on knowledge gained during the development of three approved ADCs, brentuximab vedotin (Adcetris®), ado trastuzumab emtansine (Kadcyla®) and inotuzumab ozogamicin (Besponsa®), and the many ADCs in development, this meeting addressed strategies and the latest innovations in the field from fundamental research to manufacturing.